During the past few years, wireless operators have offered mobile broadband services based on WCDMA/HSPA. Also, fuelled by new devices designed for data applications, end user performance requirements have increased. The large uptake of mobile broadband has resulted in heavy traffic volumes that need to be handled by the HSPA networks have grown significantly. Therefore, techniques that allow operators to manage their spectrum resources more efficiently are of great importance.
It is possible to improve the downlink performance by introducing support for techniques such as 4-branch MIMO, multiflow communication, multi carrier deployment, etc. Improvements in spectral efficiency per link are approaching theoretical limits. As a result, the next generation technology tends to focus on improving the spectral efficiency per unit area. Additional features for HSDPA should then provide a uniform user experience to users anywhere inside a cell by changing the topology of traditional networks. Currently 3GPP has been working on this aspect using heterogeneous networks.
A network may be homogeneous or heterogeneous. A homogeneous network is a network of base stations in a planned layout and a collection of user terminals in which all base stations have similar transmit power levels, antenna patterns, receiver noise floors, and similar backhaul connectivity to the data network. Moreover, all base stations offer unrestricted assess to user terminals in the network, and serve roughly the same number of user terminals. Examples of base stations include NodeB, eNB, eNodeB, etc. Current wireless systems that come under this category include GSM, WCDMA, HSDPA, LTE, and WiMax.
In a heterogeneous network or HetNet, in addition to the planned or regular placement of macro base stations, several pico/femto/relay base stations are deployed as illustrated in FIG. 1. The power transmitted by these pico/femto/relay base stations is relatively small compared to that of the macro base stations. For example, the transmission power from these low power nodes may be up to 2 W while the macro base stations may transmit at a power level up to 40 W. The low power nodes are typically deployed to eliminate coverage holes in the homogeneous network using macro base stations only. The low power nodes can improve capacity in hot-spots. Due to their low transmit power and small physical size, the pico/femto/relay base stations can offer flexible site acquisitions.
Heterogeneous networks can be divided into two deployment categories—co-channel deployment and combined cell. In the co-channel deployment, a low power node has a cell identifier different from that the macro node. That is, the low power nodes create different cells. But in the combined cell deployment, the low power node has a cell identifier same as that of the macro node.
FIG. 2 illustrates an example of a co-channel heterogeneous network deployment where the low power nodes create different cells. Simulations indicate that significant gains in the system throughput as well as cell edge user throughput can be realized through the co-channel deployment. One reason for the improved throughput is that the co-channel deployment provides opportunities for load balancing. In a heavy data traffic scenario, the load in the macro cell may be shared between the macro node and low power nodes. Also users with low SINR may be served by strategically located low power nodes. In short, the low power nodes can provide resources to serve users and thereby increase average user throughput of the network.
However, since each low power node creates a different cell, one disadvantage of the co-channel deployment is that a soft handover is necessary when a user equipment or UE moves from one low power node to the macro node or to another low power node. As a result, a higher layer signaling is necessary to perform the handover. In one aspect, any layer above the physical layer may qualify as the higher layer.
FIG. 3 illustrates an example of a combined cell heterogeneous network deployment. This is also referred to as soft cell or shared cell deployment, and the terms may be used interchangeably. As indicated, the low power nodes are part of the macro cell in this deployment. As such, the combined cell deployment can avoid the frequent soft handovers, and hence, can avoid the higher layer signaling.
Even though huge gains in terms of average sector throughput can be achieved with the introduction of low power nodes, the interference structure becomes more complex in heterogeneous networks. For example when a UE is connected to a low power node, individual UE link throughput may be impacted due to the interference of macro node power.
FIG. 4 illustrates an example scenario where link performance of the UE may be impacted by the macro node. The figure is applicable in both co-channel and combined cell deployments. Hence, generic term “coverage area” will be used. In the figure, two low power coverage areas served by two low power nodes within a macro coverage area are illustrated. The gray portion of the low power node coverage area is the range expansion zone. In this zone, the path loss to the macro node is higher than the path loss to the closest low power node largely due to distance differences from the zone to the nodes. But at the same time, the received power from the macro node is higher than the received power from the low power node in the range expansion zone largely due to transmit power differences among the nodes. UEs in the range expansion zone connected to the low power node may be subjected to interferences from the macro node transmissions.
FIG. 5 shows a graph of a link performance when a UE, which is connected to a low power node, experiences a strong interference from the macro node such as UEs in the range expansion zone of a low power coverage area. The interference due to other nodes is modeled as the white noise. From FIG. 5, it can be observed that there can be huge performance degradation due to the macro interference. The performance loss can be in a range of 100% at high geometries.